Usually, an electric motor drives, directly or via a gearbox, a pinion meshing with the girth gear which is arranged and supported directly on the foundation as a single pinion drive with one pinion on one side of the mill. It may also be a dual-pinion drive with one pinion on each side of the mill. There are also other drive concepts used, like ring motor and side drives. For very large mills, only ring motors are used so far. With this conventional concept, mills with a power of 9 MW (for single-pinion drives) or up to 18 MW (for dual-pinion drives) can be driven. Mills comprising a total of four girth gear pinions may be driven with a power of up to 40 MW. However, all parts of the power train need to be in a size big enough to transmit the necessary power onto the girth gear.
US 2013/0239723 A1 discloses a mill comprising a gear ring suitable for being fastened to a grinding chamber, a first reduction transmission suitable for transmitting a rotation from a first motor to the gear ring, the reduction transmission comprises a case, an output module including an output pinion and an output shaft defining an output axis. The gear ring has helical teeth, the output pinions have helical teeth, and the output module can be inserted in a single piece into the associated case in an insertion direction or removed in a single piece from the associated case in a removal direction. The driving device comprises disconnectable connecting means suitable for rotatably connecting the output shaft to a transmission element and allowing in the disconnected state a rotation of the output pinion around the output axis upon removal of the output module outside the case or during insertion of the output module into the case.
An output module comprises a shaft, bearings, a speed reduction gearing, and a helical or spur output pinion. The output pinion meshes with the gear ring and each output module is enclosed in a case. The case is properly sealed and secured to the gear ring casing.
US 2013/0008273 A1 discloses a grinder that is provided with a drive device for a crown gear which comprises a reduction transmission which is provided with a drive shaft adapted so as to be connected to a motor, with at least one output pinion being adapted so as to mesh with the crown gear and to be driven by the drive shaft and with a casing.
The reduction transmission casing comprises shafts, bearings, a speed reduction gearing and spur output pinions. The spur output pinions mesh with the spur crown gear. The casing is properly sealed and secured to the crown gear casing.
“Bradken's innovative single motor twin pinion [SMTP] grinding mill drive” by Greg Evans and Eric Hofmann, SAG Conference in Vancouver, Canada, 2001 also discloses a gearbox device which connects a motor with the pinions meshing with the girth gear. The gearbox is separated from the pinions meshing with the girth gear so that a separated gearbox for the transmission pinions and the supporting cartridge for the pinions meshing with the gearbox are provided. The pinions meshing with the gearbox are supported by the cartridge. The pinion cartridge comprises shafts, low-speed bearings, and spur or helical pinions. The pinion bearings are mounted in an eccentric cartridge and the pinions mesh with the girth gear. The pinion cartridge is properly sealed and secured to the girth gear casing. However, this also makes it more difficult to manufacture the respective housing since the holes where the input shafts are entering the cartridge and where the supporting bearing is arranged need to be manufactured very accurately. There are some solutions to these problems like excentric bearing sleeves which can be correspondingly rotated until the bearing is in the correct position. However, as more elements are to be manufactured, cost of the resulting driving arrangement increase.
“Quadrex®: A mechanical drive train solution for high-powered grinding mills” by Fabrice Lessard, et al., SAG Conference in Vancouver, Canada, 2015 also shows a device with a gearbox including the pinions meshing with the girth gear. The Quadrex® is a modular unit and comprises shafts, bearings, speed reduction gearing, and helical or spur girth gear pinions. The girth gear pinions mesh with the girth gear. The Quadrex® is properly sealed and secured to the girth gear casing.
The aforementioned solutions provide a possibility to manufacture the single elements in a smaller size and, thus, are easier to mount. Furthermore, the power for driving the horizontal grinding mills can be increased. The transmission pinions are, however, mounted together with the output pinions meshing with the girth gear in a common housing and are also supported by this housing. This housing is connected to the housing of the girth gear in which lubrication has to be provided to ensure smooth running and to avoid strong abrasion on the girth gear. The pinions driving the girth gear are mounted in housings, and the housings are directly secured to the girth gear housing. The SMTP housing contains the pinons driving the girth gear and the low-speed bearings supporting the pinions driving the girth gear. In US 2013/0239723 A1, US 2013/0008273 A1 and the Quadrex®, the housing comprises shafts, high-speed bearings, low-speed bearings, speed reduction gearing and the pinions driving the girth gear.
In a mining environment is not practical to adequately seal the girth gear or the housing. Therefore, the girth gear and the housing are contaminated with slurry or other environmental contaminants. The girth gear are designed and manufactured to accept some environmental contaminants. However, the contaminants will diminish the life expectancy of the bearings and/or speed reduction gearing.
Owing to the low operating speeds of girth gears, it is not possible to develop a hydrodynamic lubrication film. It is for this reason that girth gears require tenacious high viscosity lubricants. These types of lubricants are not adequate to lubricated low or high-speed bearings or speed reduction gearing and, thus, have a negative impact on the life expectancy of each of these components.
However, the housing of the transmission pinions needs to be specifically adjusted for supporting all the pinions. For this, the relations of the different bearings within the housing have to be adjusted to each other very accurately owing to the engagement of the pinions meshing with the girth gear.